Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 56)

Publication 1757-UM006A-EN-P - May 2002
2-12 Integrating Fieldbus into Rockwell Automation Logix System
The block-like output agent maps the ProcessLogix OP with status
parameter to the fieldbus data structure (DS-65) of the CAS_IN
parameter. It interprets the value portion in ProcessLogix terms and
converts it to fieldbus representation. The floating-point
representation is identical, in most cases, but the ProcessLogix
+/-infinity value must be converted to a fieldbus representation. If the
status of OP is “BAD”, its value must be converted from NaN to zero
(0.0) for fieldbus representation or it may retain its previous good
value, as long as the fieldbus status byte indicates “BAD”.
The CEE output agent also accepts a single BKCAL_OUT parameter
with the fieldbus data structure (DS-65) and maps it to the
BACKCALIN parameter of the PID block in ProcessLogix terms.
Like the FIM, the output agent supports both publish/subscribe and
client/server communication methods. The publish/subscribe method
allows the FIM to appear as a fieldbus device on the H1 link. The FIM
publishes the output (OP) for subscribing fieldbus device resident
blocks such as Analog Output and Proportional, Integral, Derivative
(PID) through their CAS_IN parameter input connection. This
connection is generally used when the downstream control block is in
the Cas (cascade) mode. This means that the fieldbus block's
BKCAL_OUT parameter is published by the downstream block and
subscribed to by the FIM.
The client/server method allows the FIM to appear as a computing
device on the H1 link. The FIM writes the output (OP) to be read by
fieldbus device resident blocks such as Analog Output and
Proportional, Integral, Derivative (PID) through their RCAS_IN
parameter input connection. This connection is generally used when
the downstream control block is in the RCas (Remote Cascade) mode.
This means that the fieldbus block's BKCAL_OUT parameter is written
by the downstream block and read by the FIM.
The client/server method also allows the FIM to function in a Direct
Digital Control (DDC) mode or the Remote Out mode in fieldbus
terms. In this case, the FIM writes the output to be read by the
fieldbus PID block though its ROUT_IN parameter input connection.
In turn, the PID block publishes the ROUT_OUT or back calculation
output value for the subscribing FIM.
ATTENTION
!
The ProcessLogix Control Builder application
automatically makes the appropriate back calculation
connections during configuration and the
connections are “hidden” in Control Chart views.